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Perhaps this might help somebody else:

The Mac offline installer asks for admin rights, which is absolutely not needed (and I won't be bothered if GOG possibly made a mistake in their install scripts and breaks my computer). The game is simple enough and doesn't need any installation, you should be able to copy it into any folder like ~/Applications without admin.

This is how to extract the game from the installer package and use it from any folder without the installation:

1) in Terminal, run "pkgutil --extract beneath_a_steel_sky_enUS_1_0_33348.pkg [path to extract]
2) open extracted folder, rightclick package.pkg and Show package contents
3) look for package folder, drag it out wherever you want the game and rename it to Beneath a Steel Sky.app, or you could go to Resources/game/game where the actual ScummVM game image is to use with standalone ScummVM app, which is much better than the one bundled by GOG.
5) you might possibly need to remove quarantine xattr and chown it, I did not.
Nice! Thanks for the post.

In my case the (related to point 4) the folder I wanted to extract from the installation package was "payload".